Plasticized polyvinyl butyral is widely used as an interlayer or insert in the laminated safety glass used particularly in automobiles or building constructions. The laminated safety glass used for these purposes should exhibit certain properties such as good edge stability, heat resistance and shock resistance, particularly at low ambient temperatures. The plasticized polyvinyl butryal interlayer determines the degree to which these desired properties are exhibited by the laminated safety glass.
The edge stability, or resistance to delamination of the laminated safety glass, is directly linked to the compatibility of the plasticizer with the polyvinyl butyral resin (PVB). The most typical incompatibility of the plasticizer and the PVB resin results in the exudation of the plasticizer from the plasticized polyvinyl butyral sheet and subsequent delamination of the laminated glass.
The use of mixed alkyl alkylaryl adipates as PVB plasticizers is known in the prior art, European Pat. No. 0011577. These adipates are generally represented by the following structural formula: ##STR1## wherein n is greater than or equal to 1 and the sum of p+r is greater than or equal to 1.
Suitable mixed adipates of this formula are, for example, benzyl octyl adipate, benzyl hexyl adipate, benzyl butyl adipate and benzyl decyl adipate.
The mixed adipates are useful as plasticizers for PVB resins having a molecular weight of 30,000 to 600,000, comprising 0 to 10% by weight of residual ester groups, calculated as polyvinyl acetate and 12 to 25% by weight of hydroxyl groups, calculated as polyvinyl alcohol. Mixed adipates of this type have a good compatibility with the PVB resin. Thus, laminated glasses containing these plasticized polyvinyl interlayers exhibit good mechanical properties and have excellent edge stability and good heat resistance. For certain uses, however, these mixed adipate plasticized polyvinyl butyral interlayers fail to provide the laminated safety glass with adequate cold resistance and particularly shock resistance in the cold.
The use of dialkyl adipates, such as dibutyl adipate, di-n-pentyl adipate, di-n-hexyl adipate, di-n-heptyl adipate and di-n-octyl adipate as PVB plasticizers is also known. However, these plasticizers exhibit only a partial compatibility with the PVB resin that decreases as the molecular weight of the adipate and/or the hydroxyl group content of the PVB resin increases. This necessitates either the use of resins having a low hydroxyl group content, and consequently, less adherence, or low molecular weight dialkyl adipates. Under these circumstances, the heat resistance and/or the edge stability of the laminated glass containing these plasticizers is unsatisfactory.
The use of certain carboxylic esters of glycol as PVB plasticizers is known from U.S. Pat. No. 2,229,222. According to that reference, the carbon chain of the acid contains no more than 6 carbon atoms and the plasticizers include: diethylene glycol di-2-ethylbutyrate, triethylene glycol di-2-ethylbutyrate, tetraethylene glycol di-2-ethylbutyrate, diethylene glycol di-n-hexoate, triethylene glycol di-n-hexoate, triethylene glycol di-2-methyl pentoate and pentaethylene glycol di-2-ethylbutyrate. These plasticizers are generally quite compatible with PVB and the plasticized PVB product frequently demonstrates good cold resistance. However, the compounds are costly, and when used in the amounts called for--on the order of 30% by weight in proportion to the PVB resin--lead to an expensive plasticized product.
Although it is known, according to U.S. Pat. No. 4,230,771 to use tetraethylene glycol di-n-heptanoate as a plasticizer for PVB, the other diesters derived from acids having a carbon chain of 7 or more carbon atoms are generally slightly or not at all compatible with PVB.